A focused electron beam system (ebeam) is commonly used to create or examine the microstructure of articles. A common article of interest is a silicon substrate, such as is used in the fabrication of integrated circuits. The beam is formed with electrons that are emitted from an emitter in an electron gun, and acts as a fine probe when it interacts with the substrate for examining microstructures. The performance of the ebeam is generally characterized by the spot size at a given beam current.
As depicted in FIG. 1A, a commonly used electron gun for generating and controlling the electrons in an ebeam tool is an electrostatic gun consisting of a suppressor 01, an electron emitter 03, an extractor 06, a focusing lens electrode 13, and an anode 14. The electrons emitted from the tip of the emitter 03 are selected by a beam limiting aperture 10 for controlling the beam current to the electron optical column (as depicted in FIG. 1B), consisting of the electron gun lens 13, column aperture 62, objective lens 63 and the substrate under inspection 65. The extractor 06, the beam aperture 10, the aperture holder 11, and the sandwich electrode 08 between the extractor 06 and aperture 10 are applied with the same voltage (extractor voltage). The emitter 03 and suppressor 01 are biased with negative voltages depending on the desired beam energy for the ebeam. For example, if a beam energy of 12 keV is required, the emitter is biased at −12 kV. The anode 14 is commonly grounded. The size of the beam aperture 10 inside the extractor 06 is commonly fixed in order to provide the column with a constant raw beam current within the electron beam profile 41. The beam current defined by the beam aperture 10, i, is given by:I=πJα2  (1),
in which j is a virtual source angular intensity of about one milliamperes per steradian, and the α shown in FIG. 1B is the ebeam angle defined at the virtual source plane. The final beam current delivered to the substrate 65 within the electron beam profile 42 is selected by the size of the column aperture 62 and the voltage applied on the focusing lens electrode 13. The electrons within the beam profile 42 are focused by the objective lens 63, and form a small ebeam spot on the surface of the substrate 65.
The performance of an electron gun lens is commonly characterized by a spherical aberration coefficient (Cgs) and a chromatic aberration coefficient (Cgc), because the Cgs and Cgc directly reflect the spherical aberration blur dgs (dgs˜Cgs α3) and chromatic aberration blur dgc (dgc˜Cgc α). The Cgs and Cgc are strongly related to the working distance of the gun lens. The working distance is defined as the distance between the virtual source plane and the principal plane of the gun lens. The shorter the working distance, the smaller the values of Cgs and Cgc will be. Typically, the Cgs and Cgc are given to be a few hundreds of millimeters and a few tens of millimeters, respectively. The gun lens aberration blurs are magnified via the optical magnification M of the column, and finally affect the resolution of an ebeam probe at the substrate.
However, there are some disadvantages of an electrostatic gun. For example, the large Cgs and Cgc due to a long working distance in an electrostatic gun heavily affect the resolution of an ebeam probe at the substrate, because the gun aberration coefficients Cgs and Cgc are magnified via the column optical magnification M. The total spherical aberration coefficient (Cts) and total chromatic aberration coefficient (Cts) at the substrate end in the column of FIG. 1B are calculated by:
                                                        C              ts                        =                                          C                os                            +                                                M                  4                                ⁢                                                                            C                      gs                                        ⁡                                          (                                                                        V                          LE                                                                          V                          ext                                                                    )                                                                            3                    /                    2                                                                                ,          and                ⁢                                                      (        2        )                                                      C            tc                    =                                    C              oc                        +                                          M                2                            ⁢                                                                    C                    gc                                    ⁡                                      (                                                                  V                        LE                                                                    V                        ext                                                              )                                                                    3                  /                  2                                                                    ,                            (        3        )            
in which Cos and Coc are the spherical and chromatic aberration coefficients of the objective lens 63 respectively, Vle is the landing energy voltage of the ebeam at the substrate, and Vext is the voltage on the extractor 06. It is seen from equations (2) and (3) that the total Cts and Ctc increase sharply with the increase of the column optical magnification M. In practice, a relative large M is commonly required in order to allow the optics to get a large depth of focus. Accordingly, the gun Cgs and Cgc play a critical role in affecting the resolution of an ebeam instrument.
Further, the Coulomb interactions between the electrons in the upper column (above the column aperture 62) strongly affect the resolution of the ebeam in low beam current applications. In a typical application of the electron beam inspection of a substrate, the beam currents to the substrate are divided into a number of groups. The lowest beam current group (less than about one nanoampere) is used to review an inspected substrate. The low beam current group (about five nanoamperes to about seventy-five nanoamperes) is used to inspect the substrate with a high resolution (or a high defect capture sensitivity). The highest beam current group (about five hundred nanoamperes to about one thousand nanoamperes) is used to inspect the substrate with high throughput (high scan speed of about 800 megahertz to about 1600 megahertz). In these applications, the size of the beam aperture 10 should be large enough to allow at least 1000 nanoamperes of beam current to the column. On the other hand, the size of the column aperture 62 should be small enough to select the low beam currents (<75 nanoamperes) to the substrate. This causes a large quantity of electrons to be stopped in the space between the beam aperture 10 and the column aperture. The Coulomb interactions between these electrons cause severe blur at the substrate, and prohibit the low beam current groups from achieving higher resolutions.
The advantage of the electrostatic gun depicted in FIG. 1A is that the virtual source angular intensity j in equation (1) keeps unchanged when the voltage on the focusing lens 13 is varied to adjust the column for a higher resolution and for a desired beam current to the substrate, because the lens electrical field is well-shielded by the electrode of the beam aperture holder 11.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.